Determing the f/stop of a lens

[Bosaiya]

Would someone take pity on me and point me in the direction of a formula for determining the f/stop of an unmarked lens? I've been searching for calculations and formula but keep getting the wrong kind of f/stop calculator. I know it's been posted but I can't seem to find it anywhere.

[Blueberrydesk]

Aperture diameter divided by focal length. A 25mm aperture opening on a 50mm lens would be f2. (really f 1/2, but the numerator is usually dropped)

Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size. I don't know if it would just be the diameter of the front (or rear) element.

[Bosaiya]

Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size. I don't know if it would just be the diameter of the front (or rear) element.

Thanks! Yes, a barrel lens. I'm not sure what to measure.

[Greg Lockrey]

Aperture diameter divided by focal length. A 25mm aperture opening on a 50mm lens would be f2. (really f 1/2, but the numerator is usually dropped)

Or is the lens you have a barrel lens with no aperture? The math would be the same, but I'll let someone else comment on how to determine aperture size I don't know if it would just be the diameter of the front (or rear) element.

At the focal length....

[Donald Miller]

Paul, I think you have the formula backwards. Should be focal length divided by entrance pupil diameter.

Example: 360 mm focal length divided by 40 mm entrance pupil diameter equals F9. As lens is stopped down to 16.36 mm it becomes F22.

Your barrel lens focal length divided by the front element diameter will get you near enough for usage.

[Blueberrydesk]

You're right Donald. As usual I'm thinking upside down.

[Bosaiya]

Paul, I think you have the formula backwards. Should be focal length divided by entrance pupil diameter.

Example: 360 mm focal length divided by 40 mm entrance pupil diameter equals F9. As lens is stopped down to 16.36 mm it becomes F22.

Your barrel lens focal length divided by the front element diameter will get you near enough for usage.

Now I'm finding that determining the focal length is no easy task. Nodal points and all that.

[Blueberrydesk]

Bosaiya, if you can post a pic of the lens there's a good chance someone here will recognize it and tell you what it is. Not me, of course...but someone.

[Bosaiya]

Bosaiya, if you can post a pic of the lens there's a good chance someone here will recognize it and tell you what it is. Not me, of course...but someone.

If only it were that easy - It's a cobbled together job.

I'm pretty sure it's around an f/3.x or something. I have a meter with a ground glass attachment and between that and a few other methods that won't be mentioned it seems a little faster than a known f/4.7 that I have in the same mount. I was just hoping there was an easy formula I might be able to apply. I think it's good enough to do some tests with.

[Andrew]

I was going to suggest putting the lens in front of an slr body to get a reading off the inbuilt light meter and then working back from that but you've already done something very similar...

given the exposure latitude of most films if you're correct to half an f-stop [or even a bit more] you'll never notice the error